1. Field of the Invention
The present invention generally relates to a method and apparatus for detecting ultrasound signals. In particular, the present invention relates to a method and apparatus for compensating for loss of sensitivity of a two-wave mixing interferometer caused by rapid scanning of a manufactured object or by the relative motion between the inspection system and the manufactured object.
2. Description of Prior Art
Ultrasound testing methods are non-invasive, generally non-destructive, techniques used to measure features of materials. These features may include layer thickness, cracks, delamination, voids, disbonds, foreign inclusions, fiber fractions, fiber orientation and porosity. The features influence a given material's qualities and performance in given applications. Each application places unique demands on the material's qualities including the need for differing strength, flexibility, thermal properties, cost, or ultraviolet radiation resistance. With the changing demands, more non-invasive, non-destructive testing of materials is being performed using techniques such as ultrasound testing.
Ultrasound techniques are applied in research as well as industrial settings. In research, ultrasound techniques are used to test new materials for desired features. The techniques are also used to seek defects in material that has undergone stress or environmental endurance testing. In industry, the techniques are used during scheduled servicing to inspect parts for defects. Aircraft, automobile and other commercial industries have shown increasing interest in these techniques.
As seen in FIG. 1, ultrasound testing uses a sonic energy signal generator 72 to initiate a sonic energy signal 76 about a manufactured object 74. The sonic energy signal 76 is measured by a sonic energy signal measuring device 78. In some cases, a signal analyzer 80 is used to discern features of the manufactured object 74 from the measured sonic energy signal 76.
The sonic energy signal measuring device is often an interferometer. One particular interferometer is a two-wave mixing interferometer. This two-wave mixing interferometer presents the advantages of being simpler, cheaper, and smaller than a Fabry-Perot interferometer. The two-wave mixing interferometer is also less sensitive to laser noise. Additionally, it has a much better low-frequency response than the Fabry-Perot interferometer.
The two-wave mixing interferometer operates by directing a probe beam at the manufactured object and collecting the scattered beam. As seen in FIG. 2, the scattered probe beam 12 is directed at a photo-refractive crystal 16. In the photo-refractive crystal 16, the scattered probe beam 12 in conjunction with a pump beam 14 creates an interference grating. Part of the pump beam 14 is diffracted by the interference grating in the crystal and travels collinearly with the scattered probe beam 12. In one embodiment of a two-wave mixing interferometer, the two beams go directly into a detector 22. In another embodiment, the beams are split into two polarized components by a polarized beam splitter 20. Then, the two polarized components are detected separately by two detectors 22 and 24. A halfwave plate 18 that controls the separation of the polarized components on each detector.
In the photo-refractive crystal, the probe beam 12 and the pump beam 14 interact to form a grating if the optical frequencies of the beams are similar. The grating diffracts part of the pump beam in the same direction as the probe beam. This diffracted pump beam has a phase-front nearly identical to the one of the probe beam. Interference between the two beams is then possible and can be detected by the detector or detectors 22 and 24.
One important parameter of the two-wave mixing interferometer is the grating building time. The grating building time is the time required for the interaction between the two beams to create the optical grating in the crystal. The grating building time is determined by the crystal properties and by the pump beam power.
However, the two-wave mixing interferometer has some drawbacks. The two-wave mixing interferometer is sensitive to target displacements. Two different effects related to the displacement direction can be observed. As seen in FIG. 3, if an axis 34 called “line of sight” is defined as the axis parallel to both the detection laser beam and to the detection optical axis, the target displacement perpendicular to the line of sight is the lateral displacement 38. The target displacement parallel to the line of sight is the normal displacement 40.
The sensitivity of the two-wave mixing interferometer to lateral displacement is directly related to the grating building time. If the target moves laterally, the speckle pattern changes, modifying the phase-front of the probe beam. If the grating building time is small enough, the grating building will be able to follow the phase-front changes and the probe and pump beam will continue to interfere. However, if the grating building time is too long, the quality of the interference between the two beams decreases, decreasing ultrasonic signal quality. The grating building time can be reduced by increasing the power in the pump beam. With increased power, the grating building time values may be low enough to allow the crystal grating to follow speckle pattern changes.
In the case of normal displacements of the target, the displacement induces an optical frequency change in the probe beam (Doppler shift). This difference between the optical frequencies of the probe and pump beams disturbs the grating in the crystal. This problem cannot be solved using practical levels of pump powers. The amplitude of the ultrasonic probe decreases rapidly with the Doppler shift. For example, for an optical wavelength of 1.064 μm and a grating building time of approximately 1 μs, the sensitivity of the two-wave mixing interferometer drops to nearly 0 for an apparent normal velocity of 0.1 m/s.
These two drawbacks limit the use of the two-wave mixing interferometer for laser-ultrasound inspection of complex composite parts. In one exemplary application, there are two lasers involved. One laser generator may be used to initiate a sonic energy signal and one laser may be used for detection. In laser-ultrasound, these two laser beams are scanned along the surface of the sample. The generation laser fires at discrete points. In addition, the detection laser fires at the same points. The two-wave mixing interferometer measures the signal during fractions of seconds after the generation laser fired. The detection laser duration is usually longer than the duration of the initiated ultrasonic signal. Usually, when scanning, the laser beams are not stopped at each measurement point. If the detection laser beam is scanned continuously during the measurement, even only for a few microseconds, there is a change of distance between the measurement point and the laser. This change during the measurement creates an apparent movement of the scanned object.
If the detection laser beam is scanned along the surface of composites having complex shapes, the laser beam scanning effect is similar to sample displacements. An adequate pump power can compensate for the lateral displacement of the laser beam, however, the normal displacement would rapidly render the signal useless. FIG. 4 illustrates the apparent normal displacement while scanning a part 52. The distance from the scanning mirror 56 to the point where the laser probe beam impinges the sample is X. The distance X is parallel to the line of sight of the system. The distance X changes with the scanning mirror angle and the part shape. As the scanning mirror angle varies, the distance changes. The changing distance creates an apparent normal displacement in the part 52. This apparent normal displacement causes an apparent change in frequency. This apparent change in frequency is similar to a Doppler shift. This shift reduces the sensitivity of the two-wave mixing interferometer. As a result, it is difficult to make measurements of sonic energy signals with a two-wave mixing interferometer in a rapid scanning testing system.
As such, many two-wave mixing interferometers suffer from lost sensitivity during rapid scanning. Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.